1. Field of the Invention
The present invention relates to a supercharged engine which has a high geometric compression ratio.
2. Description of the Prior Art
A variety of engines are known in the prior art in which a supercharger is employed for supercharging intake air to increase an intake air charge density in the engine as disclosed in, for example, Laid-Open Japanese Utility Model Application No. 56-171630 and Laid-Open Japanese Patent Application No. 61-19933. Types of superchargers which are known include a turbocharger driven by exhaust gases, and a mechanical supercharger driven by an engine output shaft.
In the engine having a supercharger, the exhaust temperature tends to rise excessively at the time of high supercharge particularly during a high-speed, high-load operation of the engine. In order to restrain such an excessive temperature rise and to maintain the reliability of the exhaust system, conventional engines are designed to lower the exhaust temperature with evaporative latent heat by enriching an air-fuel ratio during a high-speed, high-load operation. However, if the air-fuel ratio is enriched as such, a larger quantity of fuel than required for engine power output is supplied, thus resulting in increased fuel consumption.
In the meantime, the aforementioned lowered fuel economy might be compensated for by raising the cycle efficiency of the engine. To achieve this, a high geometric compression ratio is required. Conventional supercharged engines, however, have a drawback in that if the compression ratio is increased, knocking is likely to occur during high supercharge; therefore, the geometric compression ratio must be established at a relatively low value of 8.5 or less.